1. Field of the Invention
The present invention relates to an image forming apparatus which forms images with high image quality by properly estimating an ink temperature at nozzles of a recording head.
2. Description of the Related Art
In general, there is a known image forming apparatus which discharges ink to a recording medium such as a recording paper sheet, namely, an inkjet printer. The image forming apparatus has a recording head which discharges ink by driving an actuator constituted by a piezoelectric element or a heater element. When the actuator is driven, the actuator generates heat not only from the actuator itself but also from a driving section (drive IC) for the actuator. If the image forming apparatus is modified to employ an increased number of nozzles to increase recording speeds and to employ a line-type recording head, an amount of heat generated from the nozzles increases.
As a measure to restrict the apparatus from heating too highly, for example, Jpn. Pat. Appln. KOKAI Publication No. 2006-199021 has proposed a technique of propagating generated heat to ink by providing a eject path (ink feedback path) in an ink feed path including a recording head.
Ink used in image forming apparatus has a viscosity which decreases in accordance with increase in ink temperature, regardless of types of the ink. Therefore, if the recording head is driven by the same drive voltage as under a low temperature when the ink temperature has increased very high, an amount of discharged ink (namely, a drop volume amount) increases and thereby denses colors of images or bleeds. That is, if the temperature of a driven recording head rises high, the ink temperature of ink inside the recording head also rises. If the recording head is still driven at the constant drive voltage, image quality of formed images (or quality of printed characters) changes and, image quality cannot be maintained constant.
Hence, the recording head drive voltage need to be corrected. For example, according to Jpn. Pat. Appln. KOKAI Publication No. 2006-199021, a temperature sensor is provided in a downstream side of a recording head along an ink circulation path for cooling the recording head, to detect an ink temperature. A drive voltage of the recording head is corrected by using a detected value thereof. Further, another temperature sensor is provided in an upstream side of the recording head along the ink circulation path, to adjust the ink temperature of ink supplied to the recording head.
Usually, temperatures of a recording head or nozzles cannot be correctly estimated by a mere temperature control with use of only ink temperatures in an upstream or downstream side of the recording head.
Reasons thereof will now be described below. FIG. 11A and FIG. 11B graphically represent temperature characteristics of ink flowing through the recording head. The recording head is provided on an ink circulation path. The vertical axes in FIG. 11A and FIG. 11B represent temperatures of ink, and the horizontal axes thereof represent time from when ink supplied from an ink path in the upstream side flows into the recording head to when ink not used (or not discharged) is ejected to the ink path in the downstream side.
Between FIG. 11A and FIG. 11B, ink temperatures in the upstream side of the recording head are equal. FIG. 11A particularly represents an example in which the ink temperature in the downstream side of the recording head is high. In contrast, FIG. 11B represents an example in which the ink temperature in the downstream side of the recording head is low. Here, TC denotes an ink temperature at nozzles when the ink temperature in the downstream side of the recording head is high. TD denotes an ink temperature at the nozzles when the ink temperature in the downstream side of the recording head is low. Further, the nozzles are supposed to exist in the substantial center of a segment from a point where ink flows into the recording head to a point where the ink flows out.
As is obvious from FIG. 11A and FIG. 11B, the ink temperature in the downstream side of the recording head cannot be known by only detecting the ink temperature in the upstream side of the recording head, in both figures. Therefore, even if the ink temperature in the upstream side of the recording head stays equal, by what degrees ink changes inside the recording head cannot be known. This is because the recording head itself serves as a heating source or a heat radiation source depending on whether the recording head is driven or not driven. As ink flowing through such a recording head is ejected, the ink receives heat from the recording head in some cases, thereby increasing the ink temperature, or the recording head deprives the ink of heat in some other cases, thereby decreasing the ink temperature.
A difference in temperature of the recording head is a difference between the ink temperatures in the upstream and downstream sides of the recording head. Therefore, unless the difference in temperature is unknown, an actual temperature of the recording head (particularly at the nozzle) cannot be known. As represented also in FIG. 11A and FIG. 11B, the ink temperatures TC and TD at the nozzles depend on the ink temperature in the downstream side of the recording head. Consequently, the ink temperature at the nozzles cannot be accurately estimated only from the ink temperature in the upstream side of the recording head.
Therefore, even if an ink temperature at the nozzles is calculated (estimated) based on only one of the ink temperatures in the upstream and downstream sides of the recording head, the calculated ink temperature greatly deviates from an actual ink temperature because a temperature difference by which an ink temperature changes inside the recording head is unknown. Therefore, an ink temperature at which ink can be discharged cannot be said to have been reached. As a result, the amount of ink discharged from the nozzle becomes improper and causes a deterioration of image quality.
Hence, the present invention is directed to providing of an image forming apparatus capable of forming high quality images on recording media by estimating an ink temperature at nozzles in a recording head with improved accuracy and by accordingly optimizing an ink discharge amount of ink to be discharged from the recording head.